If you are referring to the seat belt in Booster mode, we recommend not sliding the shoulder portion of the seat belt into the belt guide as long as the belt fits on the center of the collar bone. As with any booster seat, we always recommend the parent buckle in the child and talk with them about sitting properly and not moving out of position. My son is almost 8 yrs old. He is now old enough to know better, but he is also old enough to know how to feed the seat belt back into the retractor if he does lean forward. If you have a child that cannot sit properly, they should remain in a 5 pt harness that is appropriate for their weight and height.
I hope this answers your questions. Please let me know if I can assist you further. ---------------------------------------